A procedure to evaluate coincidence-summing correction values in gamma-ray spectrometry with HPGe detectors

The spectrometric analysis of a gamma-emitter sample measured in "close geometry" can be affected by errors due to coincidence-summing effects that can occur when two or more coincident photons (real coincidences) within the resolution time of the spectrometric system are emitted. The probability that these effects occur depends primarily on the nuclide decay scheme, the sample-detector distance and detector intrinsic-efficiency whereas results independent from the sample nuclide activity. Coincidence-summing corrections of the photopeak areas are requested, in particular, when the gamma ray spectrometric analysis is aimed to efficiency or activity determination. In this work is reported a fast and simple procedure to evaluate true coincidence-summing correction values for point sources. Two HPGe detectors and various source-end cap distance were considered. The values are referred to main emission energies of some nuclides chosen among the most used calibration isotopes, fission products, activation products and environmental isotopes. The values of correction factors were quickly get by using coincidences expressions that involve only the knowledge of Full-Energy-Peak Efficiency (FEPE) and Total Efficiency (TE) values or suitable calibration curves. The FEPE and TE trends were determined by using 4th order log-log polynomial fits of experimental measurements of "single line" point sources, i.e. sources showing a single gamma emission, furnished by CEA. Alternatively, experimental Total-to Peak ratios can be also considered. Finally, in order to assess the errors in other geometries a suitable application of the known “efficiency transfer method” was used and validated by means of a comparison with experimental determinations.